1. Field of Invention
The invention relates to the field of loudspeakers in general, and in particular to high power electro-magnetic loudspeakers having vented pole pieces.
2. Description of Prior Art
A typical loudspeaker, such as that shown in FIG. 1, includes a paper, metal, or plastic diaphragm that is driven by an attached coil located in a magnetic field. The coil, commonly referred to as the voice coil, is of a cylindrical shape, is typically wound on a voice coil former and is centered in a circular magnetic field gap by means of one or two circumferential or concentric flexible suspensions, commonly referred to as the voice coil suspension. The voice coil suspension provides for attachment of the diaphragm and the voice coil former at the junction of the diaphragm and voice coil former with the top plate, solid (unvented) pole piece or frame of the loudspeaker. Alternating electrical current, when applied to the voice coil, will cause the diaphragm to vibrate, and therefore radiate acoustical energy into the air. The primary purpose of the voice coil suspension is to precisely center the voice coil in the magnetic field in two axes, so as to allow fore and aft excursions while preventing the voice coil from contacting the pole piece or the top plate. Conventional practice in loudspeaker design is to locate the voice coil suspension on the outside of the voice coil former and to have the circumference of the voice coil suspension fixed to the loudspeaker frame or magnetic circuit top plate. Another common method of voice coil suspension is to mount the voice coil suspension on the inside of the voice coil former by means of an axial bolt or screw in the pole pice. The centering methods cited in U.S. Pat. Nos. 287,896, 1,909,275, 1,991,526, 2,047,361, 2,085,721, 2,091,011, 2,123,640, 2,178,030, and 2,278,598 differ from the present invention because they do not permit an axial vent in the pole piece, as illustrated in FIG. 2.
A large air passage vent in the pole piece of the magnetic circuit is necessary for two reasons. Firstly, it relieves air pressurization in the area defined by the diaphragm, voice coil former, and pole piece under long diaphragm excursions which, if not vented during operation, will cause nonlinear acoustical distortion. Secondly, a large vent in the pole piece, on the order of one-third to one-half the diameter of the pole piece, is required for dissipating the heat generated in the voice coil under high power operation of the loudspeaker. A large vented pole piece will increase the heat dissipation capability of the loudspeaker by one-half, thereby reducing the possibility of voice coil warpage and greatly increasing the reliability of the loudspeaker. The method described in U.S. Pat. No. 2,347,799 is designed to limit the excursion of the loudspeaker and requires an additional conventional voice coil suspension to center the voice coil. U.S. Pat. No. 4,239,943 (FIG. 8) teaches an apparatus that could be used with a vented pole piece. It employs multiple lower suspensions to reduce eccentric motion of the voice coil where more direct methods of control are not possible. The system described in U.S. Pat. No. 4,239,943 cannot be used with a continuous integrally formed diaphragm as in the present invention.
U.S. Pat. No. 3,033,945 teaches a suspension that is located in the gap between the top plate and the circumference of the voice coil. Such a centering method is useful only for home entertainment loudspeakers with minimal acoustic output capability. Loudspeakers designed for high acoustic output require long diaphragm excursions which would be mechanically restrained by this type voice coil suspension. U.S. Pat. No. 3,496,307 teaches an elastic connection between the pole piece and the diaphragm. The material is selected to limit over-excursion at low frequencies and to also suppress unwanted resonances within the diaphragm. However, the major forces of suspending and centering the diaphragm are carried by an ordinary circumferential suspension attached to the diaphragm's outer edge.